It certainly would help to know about the original contract. If Hart had insisted upon even 10k as a set fee back when JW had nothing, then I have little sympathy. However, the way the track world (any many other individual activities) work is that a coach or teacher is usually donating time and effort as part of a broader job. Then when an athlete achieves something and starts to earn a living, the coach can ask to be compensated. Most arrangements(much like most salesmen have) would be a nominal base plus a percentage. In the legal world we often refer to this as "you eat what you kill"
JW is "killing" more these days, and so the meals should be better.
As an aside, in the golf world the players are earning about triple what they made 10-15 years ago. Caddies have always been paid a percentage. A few years ago Mark O'Meara couldn't bring himself to pay his caddy the set percentage because it would be so much, so he fired him.On the other hand, Tiger has no problem paying the 7-10%, which amounts to $1Million for his caddy!